Texas Instruments Incorporated AN-1500 User's Guide

C535676

Connector

P6

P1

Anode of
First LED

Cathode of

Last LED

User's Guide

SNVA175C–September 2006–Revised April 2013

AN-1500 LM3402/02HV Demo Board

1 Introduction

The LM3402/LM3402HV is a buck regulator derived controlled current source designed to drive a series
string of high power, high brightness LEDs (HBLEDs) such as the Luxeon™ I Emitter at forward currents
of up to 500 mA. The board can accept an input voltage ranging from 6V to 42V when using the LM3402.
When using the pin-for-pin compatible LM3402HV the upper bound of input voltage is 75V. The converter
output voltage adjusts as needed to maintain a constant current through the LED array. The
LM3402/02HV is a true step-down regulator with an output voltage range extending from a V
mV (the reference voltage) to a V
maintain regulated current through any number of LEDs as long as the combined forward voltage of the
array does not exceed V

O(MAX).

.

2 Circuit Performance LM3402

The LM3402 circuit and BOM have been designed to provide a constant forward current of 350 mA to a
single LED with a forward voltage of approximately 3.5V. (Typical of white, blue, and green LEDs using
InGaN technology.) When powered from a 24V ±5% input the demo board will maintain the average LED
current, IF, to within 10% of 350 mA. The ripple current, ΔiF, will not exceed 70 mA peak-to-peak.
Switching frequency for the demo board is 600 kHz ± 10% over the input voltage range of 6V to 42V.

determined by the minimum off time (typically 300 ns). It can

O(MAX)

O(MIN)

of 200

3 Circuit Performance LM3402HV

The LM3402HV circuit and BOM have been designed to provide a constant forward current of 350 mA to
a single LED with a forward voltage of approximately 3.5V. When powered from a 48V ± 5% input the
demo board will maintain IFto within ±5% of 350 mA. Ripple current will not exceed 70 mA peak-to-peak.
Switching frequency for the demo board is 250 kHz ±10% over the input voltage range of 6V to 75V.

4 Connecting to LED Array

The LM3402/02HV demo board includes a female 6-pin SIP, J1, connector as well as two standard 94mil
turret connectors for the cathode and anode connections of the LED array. Figure 1 shows the pinout of
J1. Solid, 18 or 20 gauge wire with about 1 cm of insulation stripped away makes a convenient, solderless
connection to J1.

Figure 1. Connecting to LED Array

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SNVA175C–September 2006–Revised April 2013 AN-1500 LM3402/02HV Demo Board

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1

T

D

t

D

t

SD

tDtSUt

SD

tDt

SU

t

SD

D

MIN

D

MAX

T T

DIM

I

F

f

PWM

T =

1

T

tD + t

SU

D

MIN

=

T

T + t

SD

D

MAX

=

t

SU

R

SNS

=

2

VIN - V

O

0.2 x L

IF x L + VO x t

SNS

-

x t

ON

Setting the LED Current

5 Setting the LED Current

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The default forward current I
this value the current setting resistor R

t

= 220 ns (2)

SNS

delivered to the LED array is 350 mA, typical of many 1W LEDs. To adjust

LED

can be changed according to the following equation:

SNS

This resistor should be rated to handle the power dissipation of the LED current. For example, the closest
5% tolerance resistor to set an LED current of 350 mA is 0.56 Ω. In steady state this resistor will dissipate
(0.352 × 0.56) = 69 mW, indicating that a resistor with a 1/8W power rating is appropriate.

6 PWM Dimming

The DIM1 terminal on the PCB provides an input for a pulse width modulation signal for dimming of the
LED array. In order to fully enable and disable the LM3402/02HV the PWM signal should have a
maximum logic low level of 0.8V and a minimum logic high level of 2.2V. The maximum PWM dimming
frequency, minimum PWM duty cycle and maximum duty cycle are illustrated in Figure 2. PWM frequency
should be at least one order of magnitude below the LM3402/02HV switching frequency. The interval t
represents the delay from a logic high at the DIM pin to the onset of the output current. The quantities t
and tSDrepresent the time needed for the output current to slew up to steady state and slew down to zero,
respectively. Typical response times for the standard LM3402 and LM3402HV demo boards circuits are
shown in Section 11.

(1)

D

SU

Figure 2. PWM Dimming Limits

The logic of DIM1 is direct, hence the LM3402/02HV will deliver regulated output current when the voltage
at DIM1 is high, and the current output is disabled when the voltage at DIM1 is low. Connecting a constant
logic low will disable the output, and the LM3402/02HV is enabled if the DIM pin is open-circuited. The
DIM1 function disables only the power NFET, leaving all other circuit blocks functioning to minimize the
converter response time.

2

AN-1500 LM3402/02HV Demo Board SNVA175C–September 2006–Revised April 2013

Copyright © 2006–2013, Texas Instruments Incorporated

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GND

DIM

BOOT

SW

CS

RON

LM3402/02HV

VIN

D1

L1

C

B

R

SNS

C

F

R

ON

C

IN

VIN = 6V to 42V (LM3402)

VIN = 6V to 75V (LM3402HV)

VCC

C

O1

OFF

DIM1

LED
ANODE

LED
CATHODE

1

2

3

4

5

6

7

8

(used in BOM 2 only)

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Low Power Shutdown

The DIM2 terminal provides a second method for PWM dimming by connecting to the gate of an optional
NFET, Q1. Note that Q1 is not provided on the standard BOM, and must be added for the DIM2 function
to operate. Q1 provides a parallel path for the LED current. This small NFET can be turned on and off
much more quickly than the LM3402/02HV can shutdown the internal NFET, providing faster response
time for higher frequency and/or greater resolution in the PWM dimming signal. The tradeoff in this
method is that the full current flows through Q1 while the LED is off, resulting in lower efficiency.

The logic of DIM2 is inverted, hence the LM3402/02HV will deliver regulated output current when the
voltage at DIM2 is low, and the current output is disabled when the voltage at DIM2 is high. Connecting a
constant logic high to the DIM2 will turn off the LED but will not shut down the LM3402/02HV.

7 Low Power Shutdown

The LM3402/02HV can be placed into a low power shutdown (typically 90 µA) by grounding the OFF
terminal. During normal operation this terminal should be left open-circuit.

8 Output Open Circuit

With either DIM terminal floating or connected to logic high, the LM3402/02HV will begin to operate as
soon as it has an input of at least 6V. In the case that the input is powered but no LED array is connected
the output voltage will rise to equal the input voltage. The output of the circuit is rated to 50V (LM3402) or
100V (LM3402HV) and will not suffer damage, however care should be taken not to connect an LED array
if the output voltage is higher than the target forward voltage of the LED array in steady state.
Alternatively, a zener diode and zener current limiting resistor can be placed in the positions Z1 and RZ. In
the case of an accidental open circuit at the output Z1 will enter reverse bias and attempt to pull the CS
pin voltage up to the output voltage. An internal comparator monitors the CS pin voltage and will disable
the internal NFET in this case. The result is a low power hiccup mode, designed to prevent excessive
voltage at the output and thermal stress on the inductor, internal NFET, and input voltage source.

Figure 3. Standard Schematic

SNVA175C–September 2006–Revised April 2013 AN-1500 LM3402/02HV Demo Board

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3

Bill of Materials

9 Bill of Materials

ID Part Number Type Size Parameters Qty Vendor

U1 LM3402 LED Driver VSSOP-8 42V, 0.5A 1 Texas

L1 SLF10145T-101M1R0 Inductor 10.1 × 10.1 × 100µH, 1A, 0.2Ω 1 TDK

D1 CMSH1-60M Schottky Diode SMA 60V, 1A 1 Central Semi

Cf VJ0805Y104KXXAT Capacitor 0805 100nF 10% 1 Vishay

Cb VJ0805Y103KXXAT Capacitor 0805 10nF 10% 1 Vishay

Cin C3216X7R1H105M Capacitor 1206 1µF 50V 1 TDK

Rsns ERJ6BQFR56V Resistor 0805 0.56Ω 1% 1 Panasonic

Ron CRCW08054642F Resistor 0805 46.4kΩ 1% 1 Vishay

Rz CRCW08050R00F Resistor 0805 0Ω 1 Vishay

DIM1, DIM2 160-1512 Terminal Silver 0.062” 2 Cambion

GND1, GND2, 160-1026 Terminal Silver 0.094” 7 Cambion

GND3, VIN, ISNS

/ C, Vo / A, SW

J1 535676-5 Connector Custom 6 Pins 1 Tyco/AMP

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Table 1. Bill of Materials (LM3402)

Instruments

4.5mm

Table 2. Bill of Materials (LM3402HV)

ID Part Number Type Size Parameters Qty Vendor

U1 LM3402HV LED Driver VSSOP-8 75V, 0.5A 1 Texas

L1 SLF10145T-151MR79 Inductor 10.1 × 10.1 × 150µH, 0.8A, 1 TDK

4.5mm 0.2Ω

D1 CMSH1-100M Schottky Diode SMA 100V, 1A 1 Central Semi

Cf VJ0805Y104KXXAT Capacitor 0805 100nF 10% 1 Vishay

Cb VJ0805Y103KXXAT Capacitor 0805 10nF 10% 1 Vishay

Cin C3225X7R2A105M Capacitor 1210 1µF 100V 1 TDK

Co1 C3216X7R1E225M Capacitor 1206 2.2µF 25V 1 TDK

Rsns ERJ6BQFR68V Resistor 0805 0.68Ω 1% 1 Panasonic

Ron CRCW08051303F Resistor 0805 130kΩ 1% 1 Vishay

Rz CRCW08050R00F Resistor 0805 0Ω 1 Vishay

DIM1, DIM2 160-1512 Terminal Silver 0.062” 2 Cambion

GND1, GND2, 160-1026 Terminal Silver 0.094” 7 Cambion

GND3, VIN, ISNS /

C, Vo / A, SW

J1 535676-5 Connector Custom 6 Pins 1 Tyco/AMP

Instruments

4

AN-1500 LM3402/02HV Demo Board SNVA175C–September 2006–Revised April 2013

Copyright © 2006–2013, Texas Instruments Incorporated

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